Batteries comprising a plurality of series-connected electrochemical cells are ubiquitous in transportation and industrial applications. Six-cell lead-acid batteries are commonly used for engine starting and energy storage in conventional automobiles and trucks and for energy storage in standby applications. Batteries comprising larger arrays of lithium-ion and nickel-metal-hydride cells are becoming increasingly common in hybrid and all-electric vehicles. With all such batteries, the cells have maximum capability and their properties are relatively uniformly distributed over the battery when the battery is new. As the battery ages, however, the cells deteriorate and their properties become more non-uniformly distributed. The challenge is to detect and quantify such deterioration in order to ascertain when the battery should be replaced.
In the past, lead-acid batteries always had filler caps making the electrolytes of the individual cells accessible. A strategy for detecting cell deterioration in such batteries employed a hydrometer to observe the distribution of the specific gravity values among the cells. A distribution that was sufficiently nonuniform identified a battery that should be replaced. For example, the following information can be found on the Interstate Battery website: “Check each individual battery cell. If the specific gravity varies more than 0.050 or “50 points” among the cells while the battery is at a 75% state of charge or above, then the battery is bad and should be replaced.” Unfortunately, this strategy has little value today since cell electrolytes are never accessible in AGM batteries and often not even accessible in flooded batteries.
Another earlier strategy for detecting a nonuniform distribution of cell properties was popular when the battery's inter-cell connectors were exposed. With such batteries, one could measure and compare the individual cell voltages. Cell voltages that deviated sufficiently from the average value identified a battery that should be replaced. Passing current through the battery while observing cell voltages enhanced the effect. Today, however, inter-cell connectors are not exposed, thus rendering this strategy also of little value.
Clearly, a method and apparatus that detects and quantifies cell deterioration in batteries for which neither cell electrolytes nor cell voltages are available would be desirable. The present invention addresses this need. It is based upon the important discovery that a well-known electrical circuit model best describes the battery's immittance characteristics (i.e., impedance or admittance characteristics) when the battery is new and all of its cells have nearly identical electrical properties. As the battery ages, cell deterioration sets in causing the cells' electrical properties to deviate from the norm and from one another. This deterioration can be detected and quantified by observing how well the circuit model actually “fits” the deteriorated battery. That is, how well the model predicts the battery's actual immittance at a particular measurement frequency. One advantage of this technique is that a pass/fail determination can be made without needing to know the battery's manufacturer, group size, or its electrical ratings.